Fundamental Research on Application of Magma Exploration Method

地下结构的知识是必不可少的，以了解火山的喷发机制，并提高可靠性的预测火山预测我们已经开发了数据记录系统，具有小尺寸，重量轻，高性能，并在本研究中，我们应用该系统来探索地下结构在万代火山，万代火山地震勘探研究小组由10所国立大学和日本气象厅组成，来自上述机构的76名成员参加了本次实验。我们在距山顶15 km范围内的火山及其周围布设了292个无人值守的临时地震台站，观测了1997年10月2日102时至2时12分间隔10分钟的8次爆炸的地震波。利用近2300个初至时资料反演了三维P波速度结构。主要结果如下：（1）万代火山及其邻近地区地下速度具有较大的水平速度反差。在海拔0.5km处，Numano-taira火山口下的P波速度为5.3km/s，而在Banadai火山南翼和Inawashire湖之间的区域，P波速度降低到2- 2.4km/s。(2)在Numano-taira斜接下方检测到垂直延伸的狭窄高速体。该岩体局部化程度高，水平延伸小于2km，类似于侵入岩墙。(3)这个侵入体被与过去的巨大喷发有关的马蹄形破火山口壁的凸起所包围。这表明，来自深处的堤坝状入侵是过去破坏性爆炸的来源。

The knowledge of subsurface structure is indispensable to understand an eruptive mechanism of volcano and to increase reliability of prediction of volcanic prediction We have developed the data logging system with small-size, lightweight and high performance and, in the present study, we applied this system to explore the subsurface structure at Bandai volcano, The research group for seismic exploration of Bandai volcano consisted of 10 national universities and JMA 76 members from the above institutions participated in the present experiment. We deployed 292 temporary unattended seismic stations in and around the volcano within 15 km from the summit and observed seismic waves loin 8 explosions that were fired at 10 minutes interval from 102 till 2 : 12 on October 2,1997. We determined three-dimensional P-wave velocity structure inverted from about 2300 first arrival time data The present results are summarized as follows ; (1) the subsurface velocity beneath Bandai volcano and its neighborhood has a large horizontal velocity contrast. At the elevation of 0.5 km above the sea level, P wave velocity is 5.3 km/s beneath the Numano-taira crater, while it decreases to 2-2.4 km/s in the region between the southern flank of Banadai volcano and lake Inawashire. (2) Narrow high velocity body extending vertically is detected just under the Numano-taira miter. This body is much localized feature, similar to intrusive dyke-like and has horizontal extent less than 2 km. (3) This intrusive body is encircled by the convexities of horseshoe shaped caldera wall associated with the past gigantic eruptions. This suggests that the dyke-like intrusion from depths had worked as source of the past destructive explosions.